The Brewers, who also lost Saturday, sit 2 ½ games back of St. Louis. The Phillies, who fell to Atlanta, move four games behind.
"Everyone knows what's at stake now," said Jon Jay, who delivered the game-winning hit for the Cards on Saturday. "We want to get back to the playoffs. We got a taste of it last year, and we definitely think we have the team to do it. We just have to do it. It was a big hit."
The Cardinals continue this stretch of nine consecutive games against the NL Central's worst two clubs with a Sunday contest at Wrigley Field and then three games in Houston. If the Cardinals are looking for a chance to pull away from those in pursuit of the final playoff spot, this would appear to be the opportune chance to do so.
"It seems like we get in those extra-inning games and a lot of them have not gone our way," manager Mike Matheny said. "But those guys keep pushing, kept fighting. Today was a good day."
Today's key games to watch (all times ET)
Cardinals (Lohse, 15-3) at Cubs (Germano, 2-8), 2:20 p.m. Preview >
Braves (Hudson, 15-6) at Phillies (Lee, 6-7), 1:35 p.m. Preview >
Brewers (Gallardo, 16-8) at Nationals (Wang, 2-3), 1:35 p.m. Preview >
Dodgers (Harang, 9-10) at Reds (Bailey, 12-9), 8:05 p.m. Preview >
If the postseason started today ...
This is how the teams would match up: Postseason Bracket »
To calculate a team's magic number, take the number of games it has remaining and add one. Then subtract the difference in the number of losses between that team and its closest pursuer.
A tiebreaker game will be played to determine a division winner, even if the tied clubs are assured of participating in the postseason. If a division championship tiebreaker is necessary, the head-to-head record between the clubs will determine home-field advantage. If the head-to-head record is tied, then the division record will be the next tiebreaker.
If two clubs are tied for the two Wild Card berths, home-field advantage will be determined by the head-to-head record between the clubs. If the head-to-head record is tied, then the division record will be the next tiebreaker.
Tiebreaker rules »
2012 postseason schedule
Wild Card playoff games: Oct. 5
Division Series: Oct. 6-12
AL Championship Series: Oct. 13-21
NL Championship Series: Oct. 14-22
World Series: Oct. 24-Nov. 1